Why does a function call to an uninitialized unique_ptr succeed in c++?

Question

When I executed the code below,

#include <memory>
#include <iostream>

class AAA
{
    public:
        AAA(void)
        {
            std::cout<<"Ctor"<<std::endl;
        }

        void show()
        {
            std::cout<<"Foo"<<std::endl;
        }
};

int main(void)
{
    std::shared_ptr<AAA> a; // uninitialized

    a->show();

    return 0;
}

The result comes out like this without any failure:

Foo

But I guess shouldn't a seg fault or something occur at the time of calling a->show() ?

And when I checked further, if there was a member variable, then a seg fault occurred.

#include <memory>
#include <iostream>

class AAA
{
    public:
        AAA(void)
        {
            std::cout<<"Ctor"<<std::endl;
        }

        void show()
        {
            std::cout<<"Foo"<<std::endl;
            std::cout<<mData;
        }

    private:
        int mData;
};

int main(void)
{
    std::shared_ptr<AAA> a;

    a->show();

    return 0;
}

The result comes out like this:

Foo
Segmentation fault (core dumped)

What is the reason for these differences? In particular, I wonder why the seg fault did not occur in the first case.